95 ZJ throwing codes 12, 21, 52 need advice - JeepForum.com

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post #1 of 6 Old 04-01-2013, 07:48 AM Thread Starter
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1995 ZJ 
Join Date: Mar 2013
Location: Vinemont
Posts: 3
95 ZJ throwing codes 12, 21, 52 need advice

OK, when my Jeep is cold it has a condition while driving which feels like the ignition tries to shut off for a second or two then it runs fine again. This is not constant and only does it until the engine warms up then the cel light comes on and the Jeep seems to run fine.
I checked the codes and the only ones that really show up are for the o2 sensor. Mine only has the one o2 sensor. Both codes are saying bad o2 sensor but before I go and replace it, just wondering if this could be the cause of the stumble, shut off during the engine warm up period? I was thinking that on the OBD 1 systems, the o2 normally doesnt affect the engine enough to cause it to stumble or shut off for a second or two.
I have already did the harness plug to PCM cleanup and zip tie fix so I know its not that.
Sort of acts like it just shuts off for a split second or two then runs fine again up until the CEL comes on, which probably means its going into open loop/limp mode.
My cat was busted up, so I had to remove it temporarily until I get up funds to buy a Magnaflow high flow Cat to put back on it.

Any help or advice is greatly appreciated. I love my Jeep and its my DD also and other than this problem it has been running great for me. I would like to get this fixed so I can continue to work at optimizing my fuel milage, which is my main focus right now.

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post #2 of 6 Old 04-01-2013, 10:12 AM
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2004 WJ 
Join Date: Dec 2009
Location: Dingmans Ferry
Posts: 2,284
Your not going to find anyone here that does a cat elimination setups. And this forum frowns upon doing it.
The ECM relies on the o2 and cat in adjusting the fuel trims and using the correct octane also is a must.
After replacing the o2 and cat (if you must) make sure the engine has a good tune up, the exhaust manifold isn't cracked, then disconnect the battery for 30 sec. This will reset the computer so it can relearn the setup with all the new parts.
If it still giving you cold start and driving problems, you might need to replace the capacitors in the ECM. The zip ty trick is used primarily on the 96-98 ZJs.
Here's the link

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post #3 of 6 Old 04-01-2013, 10:15 AM
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1997 ZJ 
Join Date: Dec 2006
Posts: 19,793
We do not discuss elimination of emissions equipment on this forum. It is against federal law regardless of local laws.
And just makes us all look irresponsible.

The o2 sensor is the only feedback the computer has telling it that what it's doing is correct and giving you optimum efficiency for conditions.

Do not buy parts store brand or Bosch sensors.ntk direct fit or mopar only.

'97 zj 5.2, some stuff, some other suff, and some things that even work sometimes.

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post #4 of 6 Old 04-01-2013, 12:24 PM Thread Starter
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1995 ZJ 
Join Date: Mar 2013
Location: Vinemont
Posts: 3
OK, thanks guys for the info.

Sorry if I overstepped the forum rules with my questions. I will refrain from any more questions about subjects involving cat removal.

I will just wait till I build up enough cash to replace the o2 sensor and put a Magnaflo high flow cat back on the Jeep and clear the codes then and see what happens with the o2 sensor codes.

My Jeep starts and runs fine so I see no need to mess with my ECM at this time.

My only issue is a hiccup/dead spot from time to time when my Jeep is getting up to its normal operating temperature while driving down the road.
Once it warms up and throws the CEL light it runs fine until I shut it off. This shows me that there is some sensor that is not functioning correctly while the Jeep is cold so I know its probably a sensor, possibly the coolant temp. sensor. I may go ahead and replace it first just to eliminate it as a possible cause.
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post #5 of 6 Old 04-08-2013, 10:02 AM Thread Starter
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1995 ZJ 
Join Date: Mar 2013
Location: Vinemont
Posts: 3
Thought I would post an update on my code problems.
I put a new o2 sensor on my Jeep and now its running smooth as glass without any issues.
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post #6 of 6 Old 04-08-2013, 08:09 PM
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1993 ZJ 
Join Date: Jul 2011
Location: plainville
Posts: 388
Awesome!! Had a similar issue with my daughters 98 and it was the upstream 02 sensor on hers. Bought both upstream and downstream sensors from rockauto for bout 65 bucks and they were both NTK brand. Glad you got it going!!
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